<div id="box" class="box">
    <ul class="list">
        <li class="content-part" data-n="栏目一">栏目一</li>
        <li class="content-part" data-n="栏目二">栏目二</li>
        <li class="content-part" data-n="栏目三">栏目三</li>
        <li class="content-part" data-n="栏目四">栏目四</li>
        <li class="content-part" data-n="栏目五">栏目五</li>
    </ul>
</div>
<div class="left">
    <ul id="left-list">
        <li data-n="栏目一">栏目一</li>
        <li data-n="栏目二">栏目二</li>
        <li data-n="栏目三">栏目三</li>
        <li data-n="栏目四">栏目四</li>
        <li data-n="栏目五">栏目五</li>
    </ul>
</div>

<style>
    * {
        margin: 0;
        padding: 0;
    }
    body .box {
        width: 1200px;
    }
    body .box {
        margin: 0 auto;
    }
    ul {
        list-style: none;
    }
    body .box ul li {
        height: 800px;
        background-color: silver;
        margin-bottom: 20px;
        font-Size: 30px;
        font-weight: bold;
    }
    body .left {
        posITion: fixed;
        left: 20px;
        bottom: 100px;
        width: 100px;
        height: 250px;
        top: 50%;
        margin-top: -125px;
        background-color: silver;
    }
    body .left ul li {
        height: 50px;
        line-height: 50px;
        text-align: center;
        cursor: pointer;
    }
    body .current {
        color: #fff;
        background-color: tomato;
    }
</style>

VAR oList = document.getElementById('left-list');
// 点击事件委托
oList.onclick = function (e) {
    if (e.target.tagName.toLowerCase() == 'li') {
        // 取data-n值
        var n = e.target.getAttribute('data-n');
        // []属性选择器
        var contentPart = document.querySelector('.content-part[data-n=' + n + ']');
        // 设置卷动
        document.documentElement.scrollTop = contentPart.offsetTop;
    }
}

// 页面卷动时，左侧盒子栏目背景随之变化
var contents = document.querySelectorAll('.content-part');
var lis = document.querySelectorAll('#left-list li');
var offsetTopArr = [];
for (var i = 0; i < contents.length; i++) {
    offsetTopArr.push(contents[i].offsetTop);
}
// 为了方便比较，追加无穷大
offsetTopArr.push(Infinity);
// 监听卷动
var nowFloor = -1;
window.onscroll = function (e) {
    var nowScrollTop = document.documentElement.scrollTop;
    // break的i值即为盒子数组下标
    for (var i = 0; i < offsetTopArr.length; i++) {
        if (nowScrollTop >= offsetTopArr[i] && nowScrollTop < offsetTopArr[i + 1]) {
            break;
        }
    }
    // 楼层不等，进行样式更改
    if (nowFloor != i) {
        nowFloor = i;
        for (var j = 0; j < lis.length; j++) {
            if (j == i) {
                // 当前楼层添加样式
                lis[j].classname = 'current';
            } else {
                // 去掉其他楼层的样式
                lis[j].className = '';
            }
        }
    }
}